Air pollution control system with chemical recovery

ABSTRACT

A system and process is described for treating the effluent chemicals from an air pollution control system for fossil fuel fired equipment. The air pollution control system involves the addition of magnesium-containing additives such as dolomite to the equipment followed by the wet scrubbing of the flue gases resulting in the reaction of the sulfur oxides with the additive. The treatment involves the addition of ammonium hydroxide to the solution removed from the scrubber to convert the soluble magnesium sulfate to soluble ammonium sulfate and precipitate magnesium hydroxide. The ammonium sulfate solution is removed from the magnesium hydroxide and either used for fertilizer or further reacted with calcium oxide to precipitate calcium sulfate and form ammonium hydroxide which is recycled for reaction with the scrubber effluent.

United States Patent Jonakin et al. 1 J 1972 [54] AIR POLLUTION CONTROLSYSTEM FOREIGN PATENTS OR APPLICATIONS WITH CHEMICAL RECOVERY 625,6497/1949 Great Britain ..23/1 28 [72] Inventors: James .lonakin, Simsbury;Arthur L. Plumley wapping both of Conn. Primary Exammer- Edward SternAttorney-Carlton F. Bryant, Eldon H. Luther, Robert L Olg Combllslion gii g. lllfi, Windsor, son,John F. Carney. Richard H. Berneike, Edward L.Kochey,

Conn. Jr. and Lawrence P. Kessler 7'; 1 Filed. July 31,1969 ABSTRACT 7Appl' 84633 A system and process is described for treating the effluentchemicals from an air pollution control system for fossil fuel [52] U.S.Cl ..23/122, 23/l l9, 23/l28, fired equipment. The air pollution controlsystem involves the 23/l93, 23/20l addition of magnesium-containingadditives such as dolomite [51] lnt.Cl. ..C0lf 11/46, C0lf5/40, COlc1/24 to the equipment followed by the wet scrubbing of the flue [58]Field of Search ..23/122, 128, I78, I 19, I93, gases resulting in thereaction of the sulfur oxides with the ad- 23'/20l ditive. The treatmentinvolves the addition of ammonium hydroxide to the solution removed fromthe scrubber to con- [56] References Cited vert the soluble magnesiumsulfate to soluble ammonium sulfate and precipitate magnesium hydroxide.The ammonium UNITED STATES PATENTS sulfate solution is removed from themagnesium hydroxide 932 130 8H909 Hunter 23/122 x and either used forfertilizer or further reacted with calcium 1 900 392 3/l933 Stump I"23/l28 oxide to precipitate calcium sulfate and form ammonium 2'345'6554/1944 13/128 hydroxide which is recycled for reaction with the scrubberef- 2,659,661 11/1953 Keitel ..23/122 x fluem- 2.781.245 2/1957Robertson et al. ...23/l22 X 4C] l D 3,320,906 5/l967 Domahidy ..23/178x STEAM GENERATCDR NH; OR A STACK M144 on PREHEATEIZ Kr equin-e512 Ca 54 Soups To FLY ASH wAs're 32 FERTILIZER neuron serruNG TANK 6 5dsotwsi-o NH4ON WASTE AIR POLLUTION CONTROL SYSTEM WITH CHEMICAL RECOVERYBACKGROUND OF THE INVENTION Air pollution has become a major problem inrecent years and there is currently much time, effort and expense beingput forth to devise economical schemes for reducing this pollution. Oneof the many sources of air pollution is the flue gases emitted fromfuel-buming equipment such as steam generating units. The sulfur oxides,S and 50,, are of major concern as air pollutants in such flue gases.The particulate matter such as fly ash and other dust particles alsocontribute to the pollution problem if not completely removed. Theschemes which have been developed to date to remove these obnoxious fluegas constituents have involved either or both high capital investmentand high operating costs rendering the schemes impractical.

One solution to this air pollution control problem is the system andmethod described in US. Pat. application Ser. No. 517,215 filed Dec. 29,1965, now abandoned, in the name of J. Jonakin, J. D. Sensenbaugh, andV. Z. Caracristi. Although this system will be more fully describedhereinafter, it basically involves the addition of magnesiumand calciumcontaining additives such as dolomite to the furnace where the additiveis converted to the oxide form. The additive reacts in the gas phasewith only a portion of the sulfur oxides to form magnesium and calciumsulfates and sulfites. The products of combustion containing theunreacted additive and sulfur oxides as well as the particulatesulfates, sulfites and fly ash are then wet scrubbed with water whichremoves the particulate matter and which provides an environment inwhich the reaction of the additive and sulfur oxides can progresssubstantially to completion. The slurry flowing from the scrubbercontains solid calcium sulfate, calcium sulfite, magnesium sulflte andfly ash and it also contains magnesium sulfate in solution. The priormethod of disposing of this scrubber effluent was to conduct it to largestorage ponds and allow a large portion of the water to evaporate afterwhich the remaining sludge is carried away.

SUMMARY OF THE INVENTION The object of the present invention relates tothe treatment of the scrubber slurry described above to remove thesoluble magnesium sulfate. This involves reacting the effluent solutionfrom the scrubber to precipitate a magnesium compound leaving thesulfate in solution. The sulfate solution may then be further processedto precipitate a sulfate compound and produce a solution which may berecycled for reaction with the scrubber effluent solution. These andother objects as well as the advantages of the invention will be morefully understood from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a flow diagramillustrating an air pollution control system for a steam generatorincluding a scrubber and the system for processing the scrubbereffluent.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,the invention is illustrated in conjunction with a steam generating unitwhich includes a furnace portion 12. The fuel, which may be any of theconventional fossil fuels containing sulfur compounds such as coal, oilor natural gas, is introduced into the furnace through burners 14. Thecombustion air, which has been heated in the air preheater 16, isintroduced into the furnace through ports 18. The combustion productspass through the furnace contacting the heat transfer surfaces and arethen conducted out of the steam generator and back to the air preheater16 in which a portion of the heat remaining in the combustion productsis transferred to the combustion air.

During the combustion process, the sulfur compounds in the fuel areconverted to 80 and S0 These sulfur oxides can 2 enter into reactions inthe furnace to form corrosive complex compounds on the high temperatureheat transfer surfaces and sulfuric acid on the low temperature surfacessuch as the air preheater. To prevent this low temperature corrosion,the gas temperatures are normally maintained above the acid dew point inwhich case a majority of the sulfur oxides are passed through thefurnace and emitted to the atmosphere through the stack.

The air pollution control system forming a portion of the presentinvention which is more completely described in-the prior mentioned US.Pat. application Ser. No. 517,2l5, involves the addition of magnesiumandcalcium-containing additives such as dolomite to the'furnace throughnozzles 20. The method of introducing the additive may take any desiredform; for instance, it can be mixed with the fuel prior to itsintroduction into the furnace or it can be added separately. Should theadditives be in the carbonate fonn, as they would be in dolomite, theywill be calcined in the furnace to the oxide form. These oxides willreact with the sulfur compounds S0 and S0 to yield calcium and magnesiumsulfate and sulfite. The sultites will be substantially all oxidized tothe sulfate form. These reactions will take place at least to a limitedextent during the passage-of the flue gases through the steam-generatingunit 10 which will tend to reduce the corrosion occurring on the heattransfer surfaces in the unit. Substantially all of the will be reactedbefore it reaches the low temperature surfaces thus eliminating thesulfuric acid corrosion problem. However, only about 30 percent of thetotal sulfur compounds in the flue gases will be reacted withdolomite-type materials prior to exit from the unit which isinsufficient to adequately control air pollution.

In order to complete the reaction of the sulfur compounds with theadditive and also to remove the particulate matter from the flue gasesincluding the sulfates, the effluent from the steam generator is passedto a wet scrubber 22 before it is released to the atmosphere through thestack 24. The calcium and magnesium oxides which are formed by thecalcination of the carbonates in the additive form hydroxides with thescrubber water. The S0 and 80; are only slightly soluble in the scrubberwater and they react rapidly with the calcium and magnesium hydroxidesin the scrubber water as well as with any undissolved calcium andmagnesium oxides and hydroxides. Perhapsabout 98 or 99 percent of thesulfur compounds can be removed by this process. The wet scrubber alsoremoves other particulate matter or dust from the flue gas stream suchas the fly ash and the inerts which may have been present in the rawadditive. The dust collection efficiency may be greater than 99 percentthus eliminating the need for electrostatic precipitators or other dustremoval apparatus. The wet scrubber 22' therefore serves both as a meansfor removing solids from the gas stream as well as a means for promotingthe reaction of the sulfur compounds withthe additive.

The reaction products of the sulfur compounds with the additive are allrelatively insolubleand can be readily removed from the scrubber waterwith the exception of the magnesium sulfate which is relatively solublein water. This solubility of the magnesium sulfate gives rise tothepresent invention. The effluent slurry from the scrubber 22 is processedin a clarifier 26 in which the solid sulfates andany unoxidized sulfitesas well as the fly ash are removed from the scrubber solution. A portionof the supernatant liquid from the clarifier 26 is recycled to thescrubber 22 along with fresh makeup water. This recycle has the effectof conserving scrubbing water and building up the concentration ofdissolved magnesium sulfate. The remaining liquid from the clarifier iscarried to a reactor 28. A solution of ammonium hydroxide is also addedto the reactor 28 in which the following reaction takes place:

The magnesium hydroxide will precipitate, leaving an ammonium sulfatesolution. The effluent from the reactor 28 is carried to the settlingtank 30 in which the precipitated magnesium hydroxide is separated fromthe solution. The reactor 28 and the settling tank 30 may be combinedinto a single piece of equipment in which the reaction and settling arecarried on simultaneously. The magnesium hydroxide slurry from thesettling tank 30 may be used in that form or it may be further processedsuch as in a kiln in which it is converted to magnesium oxide. The saleor other use of the magnesium compounds results in a return which lowersthe operating costs of the system.

The supernatant ammonium sulfate solution from the settling tank 30 mayeither be utilized in the production of fertilizer or may be furtherprocessed as shown in the drawing. This solution is carried to thereactor 32 to which is added calcium hydroxide or calcium oxideresulting in the following reaction:

Of course, other reactants could be added such as barium hydroxide, butcalcium hydroxide is preferred. The effluent from the reactor 32 iscarried to the settling tank 34 in which the precipitated calciumsulfate is separated from the remaining ammonium hydroxide solution.This calcium sulfate is then carried to waste along with the solids fromthe clarifier 26. The supernatant ammonium hydroxide solution from thesettling tank 34 is then recycled to the reactor 28 for reaction withthe magnesium sulfate. Additional ammonia or ammonium hydroxide is addedto this recycle so as to make up the ammonia lost from the system withthe magnesium hydroxide and calcium sulfate or by withdrawal of ammoniumsulfate. It is also obvious that the reactor 32 and settling tank 34 maybe combined into a single apparatus.

We claim:

l. A method of reducing pollution caused by the combustion ofsulfur-containing fuel comprising the steps of:

a. burning said sulfurcontaining fuel to form a gas stream containingsulfur oxides,

b. introducing into said gas stream an additive containing a magnesiumcompound selected from the group consisting of magnesium oxide andcompounds which produce magnesium oxide in said gas stream,

c. wet scrubbing said gas stream containing said additive with waterwhereby said magnesium oxide and said sulfur oxides are reacted toproduce a magnesium sulfate solution,

d. introducing ammonium hydroxide into at least a portion of saidmagnesium sulfate solution whereby magnesium hydroxide is precipitatedand ammonium sulfate solution is formed,

e. separating precipitated magnesium hydroxide from said ammoniumsulfate solution,

f. reacting said ammonium sulfate solution with a metallic hydroxideselected from the group consisting of calcium hydroxide and bariumhydroxide to produce a precipitated sulfate and an ammonium hydroxidesolution,

g. separating said precipitated sulfate from said ammonium hydroxidesolution, and

h. recycling said ammonium hydroxide solution for reaction in step (d)with said magnesium sulfate solution.

2. A method as recited in claim 1 wherein said hydroxide material instep (e) is calcium hydroxide and said precipitated sulfate is calciumsulfate.

3. A method as recited in claim 1 wherein another portion of saidmagnesium sulfate solution together with additional makeup water isemployed for wet scrubbing.

4. A method of reducing pollution caused by the combustion ofsulfur-containing fuel comprising the steps of:

a. burning said sulfur-containing fuel to fonn hot products ofcombustion containing sulfur oxides;

b. introducing into said hot products of combustion an additivecontaining magnesium carbonate and calcium carbonate which decompose tomagnesium oxide and calcium oxide when subjected to the heat of saidproducts of combustion whichwill react with said sulfur oxides; c. wetscrubbing said products of combustion containing said additive withwater whereby dissolved magnesium sulfate and precipitated calciumsulfate are formed in said scrubber water;

d. separating said precipitated calcium sulfate from said scrubber watercontaining said dissolved magnesium sulfate;

e. recycling a first portion of said separated scrubber water togetherwith additional makeup water for said wet scrubbing;

f. introducing ammonium hydroxide into a second portion of saidseparated scrubber water and thereby reacting said ammonium hydroxidewith said dissolved magnesium sulfate to form ammonium sulfate solutionand precipitated magnesium hydroxide;

g. separating said precipitated magnesium hydroxide from said ammoniumsulfate solution;

h. reacting said separated ammonium sulfate solution with calciumhydroxide so as to produce an ammonium hydroxide solution andprecipitated calcium sulfate;

. separating said ammonium hydroxide solution from said precipitatedcalcium sulfate; and

j. recycling said ammonium hydroxide solution together with additionalammonium hydroxide solution for reaction with said magnesium sulfatesolution.

2. A method as recited in claim 1 wherein said hydroxide material instep (e) is calcium hydroxide and said precipitated sulfate is calciumsulfate.
 3. A method as recited in claim 1 wherein another portion ofsaid magnesium sulfate solution together with additional makeup water isemployed for wet scrubbing.
 4. A method of reducing pollution caused bythe combustion of sulfur-containing fuel comprising the steps of: a.burning said sulfur-containing fuel to form hot products of combustioncontaining sulfur oxides; b. introducing into said hot products ofcombustion an additive containing magnesium carbonate and calciumcarbonate which decompose to magnesium oxide and calcium oxide whensubjected to the heat of said products of combustion which will reactwith said sulfur oxides; c. wet scrubbing said products of combustioncontaining said additive with water whereby dissolved magnesium sulfateand precipitated calcium sulfate are formed in said scrubber water; d.separating said precipitated calcium sulfate from said scrubber watercontaining said dissolved magnesium sulfate; e. recycling a firstportion of said separated scrubber water together with additional makeupwater for said wet scrubbing; f. introducing ammonium hydroxide into asecond portion of said separated scrubber water and thereby reactingsaid ammonium hydroxide with said dissolved magnesium sulfate to formammonium sulfate solution and precipitated magnesium hydroxide; g.separating said precipitated magnesium hydroxide from said ammoniumsulfate solution; h. reacting said separated ammonium sulfate solutionwith calcium hydroxide so as to produce an ammonium hydroxide solutionand precipitated calcium sulfate; i. separating said ammonium hydroxidesolution from said precipitated calcium sulfate; and j. recycling saidammonium hydroxide solution together with additional ammonium hydroxidesolution for reaction with said magnesium sulfate solution.